As a typical example of commercial production processes of a cellulose acetate, there has been known a process which comprises acetylating a cellulose by means of an acetylating agent such as acetic anhydride in the presence of an acidic catalyst such as sulfuric acid. The acetylation of the cellulose is a vigorously exothermic reaction, and when a reaction temperature, in particular a peak temperature (highest temperature) is high, depolymerization of a cellulose proceeds so that the polymerization degree of the cellulose acetate decreases and the quality of the product is deteriorated.
Therefore, it is significantly important to control or regulate the reaction temperature in the acetylation reaction with high accuracy for the purpose of sustaining qualities of the cellulose acetate.
Japanese Patent Publication No. 5761/1990 (JP-B-2-5761) discloses a production process of a cellulose acetate using a cellulose as a raw material, acetic anhydride as an acetylating agent, acetic acid as a solvent and sulfuric acid as a catalyst, which method is characterized by reducing a pressure in the reaction system over the whole period or in a partial period inclusive of the initial stage of an acetylation reaction, condensing a generated vapor and distilling a resultant product out of the reaction system to concentrate the reaction product. This literature mentions that, in the course of the reaction, when the pressure reduction is broken turning to an ambient pressure (atmospheric pressure) at the point that the vapor component is distilled off in a predetermined amount, the reaction temperature rises to reach a peak temperature due to a reaction heat in the acetylation of unreacted cellulose provided that the heat is radiated to some extent, and thereafter the temperature descends or decreases, and that the peak temperature can be controlled or regulated with a distilling amount of the vapor component taken as an index.
However, when an acetylation step under a reduced pressure (reduced pressure-acetylation step) and an acetylation step (post-acetylation step) which comprises increasing the reaction pressure and raising the reaction temperature due to a reaction heat of the rest of the cellulose to allow the acetylation reaction to further proceed are conducted continuously in a batch system, dispersion of the peak temperature in the postacetylation step in a process, as mentioned above, where pressure reduction control is ceased by taking the distilling amount of a vapor component as an index to raise the reaction temperature. Accordingly, the polymerization degree of a product cellulose acetate varies and stable qualities will not be expected.
This is probably because of the following reasons. That is, a cellulose used as a raw material generally contains moisture, so that hydrolysis of acetic anhydride due to the moisture proceeds in addition to the acetylation of the cellulose in the first half of the reduced pressure-acetylation step. Therefore, the producing amounts of the acetic acid and reaction heats in the reaction system vary or differ depending on the moisture contents of the material cellulose, and hence the amounts of distilled liquids (distillates) vary each other. Further, the reaction system in the first half of the reduced pressure-acetylation step is unstable because the reaction temperature rises violently and the amount of internal reflux is large, and distilled amount of the distilled liquid (a mixture of acetic acid and acetic anhydride) tends to vary or shift. Thus, according to the above mentioned process, the concentrations of unreacted cellulose in the reaction system practically differ even when the amounts of the distilled liquids are the same. It should be considered that the quantity of heat generated in the post-acetylation step varies with variation of the concentrations of unreacted cellulose, so that the peak temperature fluctuates or varies.
On the other hand, a peak temperature of a reaction can be stabilized, theoretically, by using an adiabatic reactor as an acetylation reactor, and strictly controlling the amounts of charged material, acetylating agent, reaction solvent and catalyst, and the reaction temperature. When the cellulose acetate is, however, produced commercially, a lot of instruments are required to take an enormous cost for the purpose of using a complete adiabatic system as the acetylation reactor, or precisely controlling the amounts or proportions of the acetylating agent or others and temperature.